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CSC 4103 - Operating SystemsFall 2009

Tevfik Ko!ar

Louisiana State UniversitySeptember 14th, 2009

Lecture - VII

CPU Scheduling - II

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Roadmap

• Multilevel Feedback Queues

• Estimating CPU bursts

• Project Discussion

• System Calls

• Virtual Machines

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Multilevel Queue

• Ready queue is partitioned into separate queues:foreground (interactive)background (batch)

• Each queue has its own scheduling algorithm

– foreground – RR

– background – FCFS

• Scheduling must be done between the queues

– Fixed priority scheduling; (i.e., serve all from foreground then from background). Possibility of starvation.

– Time slice – each queue gets a certain amount of CPU time which it can schedule amongst its processes; i.e., 80% to foreground in RR, 20% to background in FCFS

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Multilevel Queue Scheduling

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Multilevel Feedback Queue

• A process can move between the various queues; aging can be implemented this way

• Multilevel-feedback-queue scheduler defined by the following parameters:

– number of queues

– scheduling algorithms for each queue

– method used to determine when to upgrade a process

– method used to determine when to demote a process

– method used to determine which queue a process will enter when that process needs service

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Example of Multilevel Feedback Queue

• Three queues:

– Q0 – RR with time quantum 8 milliseconds

– Q1 – RR time quantum 16 milliseconds

– Q2 – FCFS

• Scheduling

– A new job enters queue Q0 which is served FCFS. When it gains

CPU, job receives 8 milliseconds. If it does not finish in 8 milliseconds, job is moved to queue Q1.

– At Q1 job is again served FCFS and receives 16 additional

milliseconds. If it still does not complete, it is preempted and moved to queue Q2.

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Multilevel Feedback Queues

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Determining Length of Next CPU Burst

• Can only estimate the length

• Can be done by using the length of previous CPU bursts, using exponential averaging

+

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Examples of Exponential Averaging

• ! =0

– "n+1 = "n

– Recent history does not count

• ! =1

– "n+1 = ! tn

– Only the actual last CPU burst counts

• If we expand the formula, we get:"n+1 = ! tn+(1 - !)! tn -1 + …

+(1 - ! )j ! tn -j + …

+(1 - ! )n +1 "0

• Since both ! and (1 - !) are less than or equal to 1, each successive term has less weight than its predecessor

Exercise

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Prediction of the Length of the Next CPU Burst

Alpha = 1/2, T0 = 10

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Project Discussion

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OS API: System Calls

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System Calls

Tanenbaum, A. S. (2001)

Modern Operating Systems (2nd Edition).

user space

kernel space

The system calls are the mandatory interface between the user programs and the O/S

! Location of the system calls in the Computing System

System calls

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System Calls

• Programming interface to the services provided by the OS

• Typically written in a high-level language (C or C++)

• Mostly accessed by programs via a high-level Application Program Interface (API) rather than direct system call use– Ease of programming

– portability

• Three most common APIs are Win32 API for Windows, POSIX API for POSIX-based systems (including virtually all versions of UNIX, Linux, and Mac OS X), and Java API for the Java virtual machine (JVM)

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System Calls

! All programs needing resources must use system calls

Operating system

User programs

Library functions & programs. . . fputs, getchar, ls, pwd, more . . .

. . . fork, open, read System calls rm, chmod, kill . . .

user space

kernel space

the “middleman’s

counter”

" system calls are the only entry points into the kernel and system

" most UNIX commands are actually library functions and utility

programs (e.g., shell interpreter) built on top of the system calls

" however, the distinction between library functions and system

calls is not critical to the programmer, only to the O/S designer

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Example of System Calls

• System call sequence to copy the contents of one file to another file

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System Call Implementation

• Typically, a number associated with each system call– System-call interface maintains a table indexed according to

these numbers

• The system call interface invokes intended system call in OS kernel and returns status of the system call and any return values

• The caller need know nothing about how the system call is implemented

– Just needs to obey API and understand what OS will do as a result call

– Most details of OS interface hidden from programmer by API

• Managed by run-time support library (set of functions built into libraries included with compiler)

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Dual-Mode Operation

• Dual-mode operation allows OS to protect itself and other system components– User mode and kernel mode

– Mode bit provided by hardware• Provides ability to distinguish when system is running user code or

kernel code

• Protects OS from errant users, and errant users from each other

• Some instructions designated as privileged, only executable in kernel mode

• System call changes mode to kernel, return from call resets it to user

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Transition from User to Kernel Mode

• How to prevent user program getting stuck in an infinite loop / process hogging resources# Timer: Set interrupt after specific period (1ms to 1sec)

– Operating system decrements counter

– When counter zero generate an interrupt

– Set up before scheduling process to regain control or terminate program that exceeds allotted time

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Standard C Library Example

• C program invoking printf() library call, which calls write() system call

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Solaris System Call Tracing

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Virtual Machines

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Virtual Machines

• A virtual machine takes the layered approach to its logical conclusion. It treats hardware and the operating system kernel as though they were all hardware

• A virtual machine provides an interface identical to the underlying bare hardware

• The virtual machine creates the illusion of multiple processes, each executing on its own processor with its own (virtual) memory

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Virtual Machines (Cont.)

• The resources of the physical computer are shared to create the virtual machines– CPU scheduling can create the appearance that users have

their own processor

– Spooling and a file system can provide virtual card readers and virtual line printers

– A normal user time-sharing terminal serves as the virtual machine operator’s console

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Virtual Machines (Cont.)

(a) Nonvirtual machine (b) Virtual machine

Non-virtual Machine Virtual Machine

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Virtual Machines (Cont.)

• The virtual-machine concept provides complete protection of system resources since each virtual machine is isolated from all other virtual machines. This isolation, however, permits no direct sharing of resources.

• A virtual-machine system is a perfect vehicle for operating-systems research and development. System development is done on the virtual machine, instead of on a physical machine and so does not disrupt normal system operation.

• The virtual machine concept is difficult to implement due to the effort required to provide an exact duplicate to the underlying machine

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VMware Architecture

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Summary

Hmm.

.

• Next Lecture: Process Synchronization

• Multilevel Feedback Queues

• Estimating CPU bursts

• Project Discussion

• System Calls

• Virtual Machines

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Acknowledgements

• “Operating Systems Concepts” book and supplementary material by A. Silberschatz, P. Galvin and G. Gagne

• “Operating Systems: Internals and Design Principles” book and supplementary material by W. Stallings

• “Modern Operating Systems” book and supplementary material by A. Tanenbaum

• R. Doursat and M. Yuksel from UNR